The final square should have an up exit, btw since an entrance leading to the square is already on the map. Also see the rules - last page - which has guidance about what to do if you meet dead ends with no further possibility of progress anywhere. The system is actually designed to minimise dead ends (especially compared to BASILISK!), so this is actually quite extreme.
Basically the rules state that in this situation, the ground crumbles and you fall down one level, possibly sustaining some damage.
Remember also thst exits revealed are in addition to any exits leading to rhe space in question from adjacent spaces.